The invention relates to a method of producing ingots of unalloyed and alloyed steels having an improved primary crystallization, reduced ingot segregation and a reduced content of non-metallic inclusions, wherein molten steel in an amount at least approximately corresponding to the desired weight of the ingot is poured into the mould, whereupon slag is supplied on it and this slag in turn is supplied with energy during the solidification of the steel, wherein, with a substantially constant ingot height, by supplying energy to the already molten slag corresponding to at least 120 kilowatt-hours or at least 103,200 Kcal/metric ton of ingot weight, the liquid core zone of that slag is kept at a high temperature while for forming and maintaining a layer of solid slag at the walls laterally confining the slag, cooling thereof is effected by means of a liquid. If desired, a melt electrolysis can be simultaneously effected in the liquid slag by the additional use of a direct current. (A method of this kind is described e.g. in Austrian Pat. No. 282,845 and British Pat. No. 1,254,546, respectively).
The supply of energy to the slag, according to such a method can be effected by resistance heating with the help of an alternating current by means of one or more consumable electrodes which have the same or a similar composition as the ingot. When using consumable electrodes having the same chemical composition as the molten steel solidifying in the mould, segregations over the longitudinal extension of the ingot and difficulties can occur, if the interval of solidification of the alloy is very wide and when in the course of crystallization a ledeburitic eutectic is formed. Segregation phenomena lead to a continuous change of the average composition of the material, in that during the crystallization process the segregating elements, such as C, Cr, W, V, Mo, Mn and S show a higher concentration from the bottom towards the top in the volumes of the ingot solidifying one behind the other. In very large forging ingots of unalloyed and alloyed structural steels (having a weight of over about 20 metric tons, e.g.), or in substantially smaller ingots of ledeburitic steels (having a weight starting from about 1.5 metric tons), the differences in the chemical composition over the cross-section of the ingot can be so great that they noticeably affect the technological properties.
The invention aims at preventing the above described difficulties and aims at an improvement or advantageous adaptation of the above described method in that sense that also in very large forging ingots or steels with strong segregation at crystallization, segregation-free crude ingots having a substantially constant chemical composition over their longitudinal extension are obtained.
The characterising feature of the method according to the present invention consists in that the quantitative composition of the consumable electrode(s) used is selected in such a manner that the concentration of the segregating elements in the amount of steel melted-off per time unit is lower by the amount by which its concentration per time unit increases in the remaining melt of the ingot.
In order to obtain the correct selection of the quantitative chemical composition of the consumable electrode(s), the concentration of the segregating elements in the solidifying ingot is determined from time to time during preliminary tests taken. In most cases it suffices when the calculation of the alloy content of the consumable electrode(s) is based on a mean value of the enrichment of the alloying elements in the remaining melt over a certain period of time, such as an hour, e.g.
According to a preferred embodiment, the chemical composition of the remaining melt is kept constant by an optimal adjustment of the melting-off rate, in that the supply of energy to the consumable electrode necessary for fusing it down is controlled, i.e. gradually reduced.
When producing large forging ingots having long periods of solidification and an enrichment of accompanying and alloying elements in the remaining melt of the ingot that strongly increases towards the end of solidification, it may be necessary to use two or more consumable electrodes in temporal sequence, the chemical composition of which is adapted to the enrichment of the segregating elements to be expected.
It is possible to use a number of consumable electrodes instead of one consumable electrode, e.g. when the ingots have wide diameters, which consumable electrodes have the same chemical compositions, or whose chemical compositions, if they differ among themselves, give a mean value when the alloy is equalized which corresponds to the composition required.
For equalizing comparatively low ingot segregations it is advantageous to use one single conically designed electrode instead of a number of consumable electrodes of varying compositions which are successively melted off, from which single electrode, at the beginning of the process larger amounts of steel are melted off per time unit from the electrode end having the wider diameter by using currents of a correspondingly high intensity, whereas, while the ingot continues to solidify, due to the decreasing diameter of the electrode and a reduction in power supplied, the respective required smaller amounts of steel are supplied to the solidifying ingot.